The present invention relates to a method of producing a carbon-bearing metallized iron briquette, and the resulting briquette.
Modern methods of producing steel result in large quantities of steel dusts and other wastes associated with steel production. Most steelmakers are searching for ways to recycle steel dusts. Proper recycling of steel dusts would allow steelmakers to reclaim valuable minerals otherwise lost as waste, and would lower the amount of environmentally hazardous materials which must be handled and disposed of properly.
The search for a method of recycling steel mill waste is driven by several factors. First and foremost are concerns related to the loss of valuable minerals. Large amounts of steel mill wastes are produced along with every ton of finished steel produced. The steel mill wastes contain percentages of iron, iron oxides, other metal oxide components, and carbon which are collected from the baghouse and water treatment apparatus of the steel mill. Through proper processing, the waste iron material can be directly reduced and melted in order to reclaim the valuable iron components. Of course, reclamation results in lower raw material costs to the steel mill.
Environmental concerns have also prompted the search for efficient methods of recycling steel mill wastes. Some steel mill wastes, such as baghouse dust from an electric arc furnace (EAF) are considered hazardous material, which must be treated before disposal. Costs of such treatment are extremely high. Even steel mill wastes which are not necessarily considered hazardous have high associated costs of land filling or other disposal due to the large volume of waste which is produced with each ton of steel.
Steelmakers have developed a method of recycling steel mill waste by collecting the waste, combining the waste with a reducing agent, compacting the combination into a solid agglomerate, then heating the agglomerate, thereby causing direct reduction of the iron materials within the agglomerate, and finally charging the directly reduced agglomerates to a steel making furnace. Methods of forming the agglomerate, known as a xe2x80x9cgreenxe2x80x9d agglomerate prior to being directly reduced, are well known in the art. An example of processing steel mill wastes into an agglomerate for direct production is found in U.S. Pat. No. 4,701,214 to Kaneko, et al., which describes a method of mixing iron oxide dust or iron ore fines with finely divided coal and a binder to form a mixture, agglomerating the mixture by compacting, pelletizing, or briquetting the mixture to form agglomerates or pellets, introducing the pellets to a rotary hearth furnace to pre-reduce the iron in the pellets, introducing the pre-reduced pellets into a smelting reduction vessel as the metallic charge constituent, introducing particulate carbonaceous fuel and oxygen to the smelting reduction vessel through the bottom of the vessel to react with the melt or bath within the vessel, reduce the iron to elemental iron and form an off gas containing CO and H2 introducing the off-gas into the rotary hearth furnace as process gas to pre-reduce the pellets therein, and drawing off the hot metal from the smelting reduction vessel.
The most advanced method of utilizing agglomerates of iron oxide fines to form a directly reduced charge to a steel furnace is seen in U.S. Pat. No. 5,730,775 to Meissner et al. which describes a method and apparatus for producing direct reduced iron from dry compacts composed of iron oxide and carbonaceous material by feeding compacts no more than two layers deep onto a hearth and removing all the volatiles and metallizing the compacts by exposing said compacts to a radiant heat source at a temperature of from about 2400xc2x0 F. to about 2600xc2x0 F.
To form the green agglomerates of the prior art, iron containing dust and/or iron ore is combined with a reducing agent, usually a carbonaceous material such as coal or coke. The agglomerate material may be wetted or dried, depending on process conditions. Finally, a binding agent is added to the mix before the mixture is compacted into a briquette.
The success of the recycle of steel dust through the direct reduction of steel dust green agglomerates depends heavily upon the quality of briquette formed prior to direct reduction. It is essential that the briquettes retain their physical integrity throughout their transit from the point of entering the direct reduction furnace to the point of entering the steel making furnace. If the briquettes fracture or disintegrate during direct reduction, then the broken fragments are subject to rapid reduction with subsequent oxidation. In the worst case, fragmented agglomerates will be reoxidized to FeO. Those agglomerate fragments which are not lost upon transfer of the agglomerates from the direct reduction furnace to the steel making furnace tend to rapidly reoxidize and melt into the slag upon injection into the steelmaking furnace or to be sucked immediately out of the steel making furnace by the off-gas containment system. Thus, loss of agglomerate material as broken fragments or dust dramatically decreases the efficiency of the steel dust recycle system.
To prevent fragmentation of the agglomerate, binders are added to the material. Choice of a binder for use in green agglomerates is often a tradeoff between cost and detriment to downstream processing. Binders traditionally used in agglomerate formation are sodium silicate, 1% lime and 3% molasses, pitch based binders, and cement. Sodium silicate produces agglomerates that are known to become weak or decrepitate upon heating, and the sodium silicate decomposes into unwanted alkali compounds, which may cause refractory damage within the furnace. Cement binders tend to increase the relative gangue content such that the slag level in the subsequent melting step becomes prohibitively high. Lime/molasses combinations and pitch based binders have acceptable performance but are comparatively costly.
There exists a need for a binder and process of utilizing a binder for steel dust agglomeration which is low in cost and results in a green agglomerate with improved crush strength, thus avoiding fracture of the agglomerate during the direct reduction process or related transportation. There is a further need for a binder and process of utilizing the binder which minimizes any downstream environmental impact and minimizes any other adverse effects on the steel making process.
It is therefore an object of the present invention to provide a process for making a strong agglomerate for further processing into carbon-bearing steel.
Another object of the invention is to provide a carbon-bearing direct reduced iron agglomerate having a metallization of at least 40%, and preferably greater than 80%, with improved strength.
The invention is a method of making metallized iron agglomerates by combining iron/steel particles and a reductant material with a cellulose fiber binder material, compacting the combination to form a solid agglomerate, and reducing the iron portions of the agglomerate in a direct reduction furnace. The cellulose fiber binder material provides an agglomerate having improved strength and lower overall cost than comparable agglomerates using binders known in the art.
The cellulose fiber material may be derived from any suitable source of cellulose fiber, and is preferably derived from waste materials such as paper, cardboard, wood scrap, bagasse, or municipal waste. Iron particles are received from waste streams of the steel making process, including baghouse dust and particulate matter from broken briquettes and pellets. Additional virgin iron components may also be added to the mixture. A reducing agent, preferably pulverized coal, is added if needed for proper reduction of the agglomerate.
The agglomerate may be a briquette formed by roll briquetting, a pellet formed by disk/drum pelletizing methods, extrusion, or other know methods of agglomerate preparation. The agglomerates are heated in a furnace for a period of 6 to 20 minutes, resulting in a very strong, carbon-containing treated product, which is extremely well suited as a feed material to an ironmaking or a steelmaking furnace.